1. Field of the Invention
The present invention relates to a semiconductor laser device, and particularly to a semiconductor laser device which is equipped with a radiating body and performs a high-power operation.
2. Description of the Related Art
There has been a demand for a high-power operation in a semiconductor laser device used as a light source for optical information processing, a signal source for optical communication, or a light source for excitation of a fiber amplifier. Even as a solid-state laser used to weld or cut a metal, e.g., an exciting light source for a YAG laser, the semiconductor laser device needs to have a high-power operation. An improvement in kink level and a reduction in aspect ratio are essential to such a high-power operation.
As a well-known example of a conventional semiconductor laser device, there has been shown a configuration wherein a refractive index distribution as viewed in the thickness direction is made asymmetric as viewed from an active layer in a semiconductor laser having a ridge waveguide to thereby prevent the occurrence of high-order modes responsible for kink occurrence, thus enabling a high-speed operation, and a light intensity distribution of light waveguided by making a refractive index of a lower cladding layer greater than that of an upper cladding layer is shifted to the substrate side upon the upper and lower portions of the active layer to distribute lots of light to the lower cladding layer, thus resulting in a reduction in aspect ratio (see, for example, Japanese Patent Laid-Open No. Hei 11(1999)-233883, paragraph number [0017] and FIG. 2).
Further, as a well-known example of another semiconductor laser device, there has been disclosed a configuration wherein a high refractive index guide layer is provided on a cladding layer on the reacting body side to thereby pull a light distribution to the cladding layer on the radiating body side on the whole, whereby an optical density in the vicinity of an active layer is reduced, a COD (Catastrophic Optical Damage) level is enhanced and high powering is made (see, for example, Japanese Patent Laid-Open No. Hei 7(1995)-38193, paragraph number [0014] and FIG. 1).
Furthermore, as a well-known example of a further semiconductor laser device, there has been disclosed a configuration wherein at least one of cladding layers provided in contact with an active layer of a quaternary semiconductor laser such as AlGaInP that performs short-wavelength oscillations is constituted of a quaternary cladding layer of a superlattice structure based on periodic laminating of thin-film semiconductors of ternary mixed crystals such as AlInP and GaInP, whereby the dimension of each mixed crystal is lowered to reduce scattering due to disordering in the mixed crystals and enhance thermal conductivity (see, for example, Japanese Patent Laid-Open No. Hei 7(1995)-170017, paragraph numbers [0011] through [0013] and FIG. 1).
However, a semiconductor laser device with a high-power operation encounters an important problem of how to effectively conduct heat generated in a laser chip to a radiator or radiating body. Therefore, a thermal conductivity distribution of materials constituting a semiconductor laser for conducting the generated heat to the radiating body, as well as a mere improvement in kink level and reduction in aspect ratio become an important problem.
On the other hand, in a compound semiconductor used in the semiconductor laser, its composition is closely related to a refractive index and also even to thermal conductivity.
According to M. A. Afromowitz, “Thermal conductivity of Ga1-xAlxAs alloys”, J. Appl. Phys., Vol. 44, NO. 3, March 1973, pp. 1292–1294, the thermal conductivity becomes lower as an Al composition ratio approximates 0.5 in the case of AlGaAs. Therefore, according to how to take or determine refractive indices of a cladding layer disposed on the radiating body side and a cladding layer disposed on the non-radiating body side, this might be a configuration disadvantageous to the transfer of heat to the radiating body.